To keep it straight, the guys at Citrix removed some drivers in the new release of Xenserver.
My test machine is a AMD X2 with a nforce 5 board and an AMD74xx chip on it for the P-ATA connections. In addition i've build up a software raid 5 with S-ATA drives for the SR where the VMs are stored and the system is on two software mirrored P-ATA drives. The boot from software raid in xenserver is allways a little tricky, not supported but still not too complicated (Dmitry Komarov wrote this together).
If your fresh upgrade or installation will continue to reboot after the initial splashscreen on and on again, boot from a live-linux (e.g. knoppix), build up a new init-ramdisk as Dmitry wrote in his howto, add up the module (in my case it was the amd74xx.ko) and be sure that the mkrootdev points correctly to your disk, the installation may have done this wrong (e.g. hde1 instead of hda1).
This was the first upgrade with 'problems' since 3.0 ...
The team released the new version 5.0 of Xenserver (http://www.xenserver.com/). The first new thing you get see after the installation is a curses menu in wich you do the basic tasks without need of cli or the XenCenter. The upgrade from a 4.1 system works just fine. If you still have 4.1 or 4.0 servers you will enjoy the option to install the 4.1 XenCenter for compatibility. I'm going to test the new HA-features later these days.